Make it:
<asp:textbox id="txtBox" runat="server" xmlns:asp="#unknown"></asp:textbox>
<asp:requiredfieldvalidator id="rfv1" runat="server" errormessage="Provide District" controltovalidate="txtBox" validationgroup="vCheck" xmlns:asp="#unknown"></asp:requiredfieldvalidator>
<asp:button id="btnAdd" runat="server" text="Add" onclick="btnAdd_Click" validationgroup="vCheck" xmlns:asp="#unknown" />